A new passive ability, e.g. Teacher or Life Guard Phobie
“No running!” Restricts enemy movement in their field of influence. In the tiles directly next to the unit you cannot move in the same turn.
So you can walk up to them (and attack). However, you couldn’t move next to it, then move again. When next to it, you could move away, or move to the next tile around it, but not get all the way round. I think tt would need to be fairly tough to be of any use, but would allow you to corner your opponents units and limit the movement options of fast moving units like Snowball.
If the ability was given to a higher key unit then it could cast the effect 2 tiles away, or even along line of sight.